This invention concerns an improved ruthenium-catalyzed process for dimerizing alkyl acrylates to dialkyl hexenedioates. The latter compounds are readily convertible to adipic acid (hexanedioic acid) by hydrogenation and subsequent hydrolysis. Adipic acid is used to produce condensation polymers such as nylon 66.
U.S. application Ser. No. 485,943, a continuation-in-part of U.S. application Ser. No. 348,364, now abandoned, discloses a process for making dialkyl hexenedioates comprising dimerizing an alkyl acrylate of the formula H.sub.2 C.dbd.CH--COOR, wherein R is a straight chain alkyl group, in the presence of an alcohol and a ruthenium compound treated with one or more metals selected from the group consisting essentially of zinc, manganese, iron, cobalt, and copper, said ruthenium compound being in an oxidation state greater than zero, there being present optionally a phosphine and optionally a carboxamide.
Alderson, U.S. Pat. No. 3,013,066, and Alderson, Jenner, and Lindsey, J. Am. Chem. Soc., 87, 5638 (1965), disclose the reaction of methyl acrylate in the presence of ruthenium chloride and methanol to give a mixture of products that includes dimethyl 2-hexenedioate.
Canadian Pat. No. 796,775 discloses the dimerization of alkyl acrylates in the presence of a mixed ruthenium compound-zinc catalyst, a hydrogen donor, and a phosphine.
U.S. Pat. No. 3,322,819 discloses dimerization of methyl methacrylate to a nonlinear product, dimethyl 2,2,4-trimethylglutarate, by heating with ruthenium carbonyl in the presence of water and carbon monoxide under pressure. In Example 2 the ruthenium compound that is charged is ruthenium trichloride, and the ruthenium carbonyl, Ru(CO).sub.5, is made in place. Ruthenium carbonyl is a compound of zerovalent ruthenium. This patent shows that a lower-valent ruthenium compound made by reducing RuCl.sub.3 does not necessarily give linear products in the dimerization of an acrylic-type ester.
Japanese Patent Publication No. 52-105115 discloses nonlinear acrylic esters dimerized with a catalyst that is a mixture of a phosphorus compound (usually a phosphine); a compound of a metal of Group I, II, III, VI, VII, or VIII; and a divalent sulfur compound. In a list of 49 exemplary metal compounds, palladium chloride is the only compound of a Group VIII noble metal.
French Patent Publication No. 2,211,443 discloses linear derivatives of monounsaturated dicarboxylic acids of formula ROC--CH.sub.2 --CH.sub.2 --CH.dbd.CH--COR (where R is lower alkoxy, aryloxy, acyloxy or aminated optionally substituted) and corresponding derivatives are prepared by dimerization of unsaturated carboxylic acid derivatives of formula CH.sub.2 .dbd.CH--COR (where R is as above) using solution Pd(II) complex catalyst and a quinone co-catalyst, the reaction being effected at 0.degree. to boiling point of the reaction medium and under normal pressure.
U.S. Pat. No. 3,548,021 discloses the oxidative dimerization of isobutylene to para-xylene using a Group VIII noble metal in an elevated oxidation state and a redox agent and oxygen to maintain the elevated oxidation state of the Group VIII metal.
U.S. Pat. No. 3,956,358 discloses the dimerization of acrylic esters to nonlinear esters of 2-methyleneglutaric acid in the presence of a catalyst comprising certain Group VIII metal halides, but not Ru halides.
U.S. Pat. No. 3,946,066 discloses a heterogeneous ruthenium catalyst for the dimerization and hydrodimerization of olefinically unsaturated compounds by co-precipitating a ruthenium compound with an aluminum oxide and/or hydroxide.